Sunnyvale and Los Altos, CA Tornadoes
May 4, 1998

Updated 1/25/03

ABOUT THIS WEBSITE

This Web Site will be in constant evolution. The opinions
on the storm have not been subjected to peer review in the refereed
literature and are the opinion of John Monteverdi alone. They
represent only a current working hypothesis on this very interesting
case and are in a state of constant evolution as new evidence
comes in.

ALL PHOTOS AND GRAPHICS ON THIS SITE ARE PROPRIETARY. ALTHOUGH
THEY MAY BE USED WITHOUT PERMISSION FOR EDUCATIONAL PURPOSES,
EXPRESS PERMISSION MUST BE OBTAINED FROM THE OWNERS FOR ANY OTHER
USE.

Please contact John
Monteverdi if you have any additional video footage which
shows the tornado or the developing funnel.

Photo Shot By Greg Yannazzo and Published in the
Chronicle 5/5/98

Estimate that the view is towards the northeast. Video
shows that this feature was rotating clockwise (anticyclonic).
There is now mounting evidence that this indeed was the Sunnyvale
F2 tornado. Analyses of radar and other videos taken by citizens
suggest that the Sunnyvale tornado was anticyclonic and, possibly,
multiple vortex. This image suggests a double funnel, with an
additional nearly horizontal small funnel off to the left
about half-way up the main lowering. The image suggests that
another funnel cloud was present on a more distant lowered cloud
base.

NEW!!

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New evidence and damage reports show that the second tornado
(which hit Los Altos about 10 minutes after the Sunnyvale tornado)
caused substantially more damage than previously thought. In
addition, the Los Altos tornado caused one serious and several
minor injuries. Post-analysis of damage suggests that the Los
Altos tornado was a high-end F1 or an F2.

Forming Funnel.

Contraction and spinup.

Between 0425 PDT and 0432 PDT, 5/4/98, an outflow boundary
encountered a developing thunderstorm in the Sunnyvale area.
Both analyses of radar and hodograph information suggest
that this storm had a clockwise rotating updraft through a relatively
deep portion of the storm. As the outflow boundary (with
a spin around the horizontal axis) encountered the storm,
it probably was tilted upwards. Lateral contraction and
vertical stretching would "spin up" the rotation into
strong motion, visible as a funnel cloud if cloud material was present.

The picture above left (taken by Karl Anderson) shows both
the forming funnel and another anticyclonic vortex visible on
its left (north) side. Picture is taken towards the south. It
probably was taken at the same time as the larger picture
above. (Click on image to see larger version)

The picture above right (also taken by Mr. Anderson) shows
the intensification of the funnel, and propogation downwards.(Click on image to see larger version)

Anticyclonic Rotating Funnel

Anticyclonic striations clearly visible. This picture
is roughly simultaneous with the tornado damage in Sunnyvale.

Looking Up The Throat of The Vortex

(Images Courtesy of D. Misaki)

These three remarkable images were shot from directly underneath
the funnel shown above. Foreshortening has made the funnel
itself appear like a dimple.

Click on image to see full-sized version.

Rotation is characterized relative to the ground, as if one
is looking down on the motion. Since the images are shot looking
at the vortex from below, at first glance it appears as if the
vortex were rotating counterclockwise. However, since rotation
is characterized as the motion around a vertical axis perpendicular
to the ground, as if the observer were looking down, these photos
provide striking corroboration of the clockwise (anticyclonic)
nature of the rotation.

The full-sized version shows flecks which, in my opinion,
are chunks of debris whirling up and around the vortex.

Click on image to see full-sized version.

The last image here has more of a three dimensional look,
with shadowing highlights. This is probably because at this point
the funnel had extended downward considerably, and probably was
at the stage of the last Anderson picture shown above.